The invention relates to controllable, reproducible, nonsurgical alteration of the corneal curvature in the human eye.
Refractive errors of the eye such as hyperopia, myopia, and astigmatism are widespread in the human population. The cornea which accounts for most of the refractive power of the eye comprises transparent avascular tissue that forms the anterior portion of the eye. It resides in the sclera at the limbus. The transparency of the cornea is due to its uniform structure, avascularity, and deturgescence, which is the state of relative hydration of the corneal tissue. The average adult cornea is about 0.65 mm thick at the periphery and about 0.50 mm thick in the center. From anterior to posterior, the cornea has the following five distinct layers: the epithelium, Bowman's membrane, the stroma, Descemet's membrane, and the endothelium.
The present invention concerns hydrothermal shrinkage of collagen fibers present in the stroma. The corneal stroma accounts for about 90% of the corneal thickness; it is composed of intertwining lamellar fibers that are about 1 .mu.m wide and run almost the full diameter of the cornea. The lamellar fibers run parallel to the surface of the cornea and by virtue of their size and periodicity are optically clear. Collagen is a protein found in connective tissues of many organs of the human body including the corneal stroma. The connective tissue of the corneal stroma possesses high transparency of cross-oriented individual sheets, or lamellae of collagen, with a high water content and small content of protein and mucopolysaccharides. The intermolecular cross-links provide the collagen fibers with unique physical properties of high tensile strength and substantial elasticity. The extracellular matrix of the corneal connective tissue consists of complex macromolecules, the biosynthesis of which involves several specific reactions that are often under stringent enzymatic control. The cross-linking of collagen fibers can be inhibited by supplying energy to the matrix. The net generation of collagen connective tissue is then dependent on the precise balance between the synthesis and degradation of the above mentioned enzyme.
The hydrothermal shrinkage property of collagen fibers has been recognized for many years. At increased temperatures, the collagen ultrastructural stabilizing cross-links rupture resulting in immediate contraction of the fibers to about 1/3 of their original linear dimension. At the same time the caliber of individual fibers increases; however, the structural integrity of the connective tissue is maintained. The shrinkage changes the overall shape of the cornea and thus changes the refractive power of the eye; this is utilized in thermokeratoplasty. This corneal recurving procedure requires a predictable collagen shrinkage and thus predictable change in the cornea shape. This should be achieved without damaging either Bowman's membrane or Descemet's membrane. One disadvantage of the above described thermal reshaping of the corneal profile could be rapid replacement of contracted collagen fibers by new mature collagen fibers of original length. This replacement is most pronounced in traumatic injuries of the eye. However, if atraumatic collagen shrinkage is achieved, it is believed that a protracted or permanent recurving of the cornea occurs.
In the past, various thermokeratoplastic techniques have been suggested. Shrinkage of the collagen fibers was achieved by applying RF current, inserting a hot microwire into the stroma, heating appropriate areas of the stroma using laser energy, or placing a surface of a hot instrument onto the eye surface. Many of these methods pose a high risk of damage to the epithelium and Bowman's membrane on the anterior side of the cornea, as well as Descemet's membrane and the endothelium on the posterior side of the cornea. Thus, it is very important to precisely control the amount of heat applied to the stroma. Sufficient heat must be delivered to cause permanent fiber shrinkage. However, if too much heat is applied, then permanent damage to Bowman's and Descemet layers can occur.
Different types of corrective procedures can be performed by selectively heating the stroma and causing selective shrinkage of the stromal collagen. Hyperopic corrections are achieved by causing shrinkage of the collagen in a ring-shaped pattern about the optical axis of the eye. Large hyperopic corrections are usually achieved by applying several concentrically arranged ring patterns. Astigmatism can be treated by applying accurate segments of the full treatment rings used for spherical hyperopia with the arcs centered on the flat meridian of the cornea. Myopic corrections can be achieved by central application of a focused energy beam in order to flatten the corneal shape or by application of radial patterns. In an eye which has several refractive errors a combination of several patterns can be used. Thus, it is necessary to select an appropriate geometric pattern corresponding to the shape of the corneal curvature prior to the laser thermokeratoplasty and deliver heat precisely to the selected locations.
Currently, laser thermokeratoplasty is performed by applying a handpiece to the corneal surface in order to irradiate the cornea. To denote the appropriate locations, a ring marker has been used to mark the cornea with a dye. Then, the ophthalmologist positions the handpiece on the marked site and irradiates the cornea. A series of focused exposures are made sequentially on the marked sites. The introduced radiation is focused to a depth of less than 450 .mu.m and is absorbed in the stroma. The plurality of focused conical exposures creates reformation of the cornea. Even though marking of the exposure sites using the dye gives some precision and reproducibility to this corrective procedure, the result of the laser thermokeratoplasty depends to a great extent on the skill of the ophthalmologist performing the procedure. Furthermore, since desired results of the laser thermokeratoplasty depend on appropriate energy delivery to the number of predetermined sites on the eye surface, and on the skills of the ophthalmologist, laser thermokeratoplasty, as currently performed, requires a high degree of tactile skill.
In summary, there continues to be a need for a surgical device and procedure which can deliver thermal energy to precisely defined locations in the stroma, for an exactly controlled amount of time, performed in a standardized manner, very quickly and without causing damage to the cornea.